1. Field of the Invention
The present invention relates to a process, for hydrodesulfurization of sulfur-containing diesel gas oil, which comprises a specific combination of specific hydrogenation steps.
2. Description of the Related Art
The cracked diesel gas oil obtained by cracking the heavy oil or the straight run diesel gas oil obtained by distilling crude oil contains sulfur compounds, and the amount is in a range from 1 to 3 wt % as sulfur. When the diesel gas oil containing sulfur compounds is used as a diesel fuel, sulfur compounds will be exhausted in atmosphere as SOx and the environment will be polluted.
Therefore, these diesel gas oils are used as a fuel usually after being hydrodesulfurized to remove sulfur compounds. It is stated that the permissible value for amount of sulfur included in a diesel fuel should be 0.05 wt % or less in the JIS (Japanese Industrial Standard), and large-scale desulfurization arrangements have been constructed to achieve this value and are used. In addition, it is said that it is necessary to decrease the amount of sulfur further with a view to installing a purification catalyst, which reduces NO.sub.x in an automotive exhaust gas, into a diesel car in the future and using a part of the automotive exhaust gas again by circulating it as a part of a diesel fuel. This system is called an EGR system (EGR: Exhaust Gas Recirculation).
A catalyst which consists essentially of cobalt or nickel, and molybdenum supported on an alumina carrier has conventionally been used for the desulfurization of diesel gas oil so far. However, conventional catalysts have problems in that they can hardly desulfurize 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene, and therefore, in order to lower the sulfur content of the product to the level of 0.05 wt % or less, it is necessary to raise the reaction temperature and the reaction pressure to a very high level, so that the construction costs of the arrangement and the drive costs increase.
As for a process for improving the desulfurization activity for sulfur compounds, a catalyst whose carrier contains phosphorous and boron has been reported in Japanese Unexamined Patent Publication (Kokai) No. 52-13503, and a catalyst to whose carrier zeolite was added has been reported in Japanese Unexamined Patent Publication (Kokai) No. 7-197039. These catalysts have Br.PHI.nsted acid sites and, thus, exhibit high ability to isomerize a methyl group of dimethyldibenzothiophene and to hydrogenate a phenyl group, and high activity to desulfurize 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene.
However, catalysts whose carriers contain phosphorous, boron or zeolite have drawbacks in that their desulffirization activities for alkylbenzothiophenes and dibenzothiophenes without 4- or 6-alkyl substituent, such as dibenzothiophene, and 1-, 2- or 3-methyldibenzothiophene are inferior to those of conventional catalysts consisting essentially of cobalt and molybdenum on an alumina carrier (F. van Looij et al. Applied Catalysis A: General 170, 1-12 (1998)). Moreover, said catalysts have further drawbacks in that, as they have Brinsted acid sites, they may easily cause a coloring of the oil product and when they are used for an olefin-containing feedstock oil or are used at a high temperature of 350.degree. C. or higher, thiols and sulfides are occasionally generated to decrease the desulfurization ratio. In addition, they have another problem in that olefin elements in a feedstock may be polymerized at Br.PHI.nsted acid sites to generate coke and the deactivation of catalyst may be accelerated. Even if olefins were not included in a feedstock, if sulfur compounds were desulfurized with said catalysts, olefins would be generated in situ, and it would cause an extraction of coke. This can be understood from the view that a coking speed, when thiophene flows into said catalyst, reaches ten times the coking speed reached when olefins or aromatic compounds flow into the catalyst (Catalysis Review, 24, (3), 343 (1982)).
Thus, as these prior arts have many problems and they do not achieve an effective manufacturing of excellent diesel gas oil with low sulfur content when used for the deep desulfurization of diesel gas oil feedstock.